This invention pertains to sound masking and more particularly to apparatus for digitally generating masking noise signals.
Modern day architectural design emphasizes open plan office landscaping wherein mobile partial height partitions are utilized to obtain maximum use and flexibility of the available space. From an acoustic point of view there is the unavoidable disadvantage associated with open plan landscaping in that there is no conversational privacy between work zones. Although the problem of reducing speech intelligibility is difficult, acoustic engineers have utilized combined techniques to successfully obtain a solution. Partial height barriers and high absorptive ceiling tiles used concurrently have a limited effectiveness. Therefore there is a need for an additional technique to achieve the required privacy.
Such a technique adds an unobtrusive steady background "masking noise" to the work area. In order for the background noise to be an effective masker it must meet several requirements. In particular it must be an electronically derived noise since such noise is inoffensive to the ear while at the same time providing the desired speech privacy to the masked zone. More specifically such noise must be a steady and continuous broadband noise having a selected frequency spectrum shape and controllable volume. These criteria negate a random approach such as relying on normal office activities, air conditioning, music and the like to properly achieve speech unintelligibility. Hence, currently the utilization of electronic noise fed into an array of speakers has become an integral consideration of current office design.
Heretofore electronic noise was generated by exploiting or amplifying the inherent thermal noise in solid state devices such as diodes and transistors. However, this type of noise generation has several drawbacks. In particular there is an output sound instability due to sporadic shot noise in the solid state devices. Secondly semiconductor devices have characteristics which change over long periods of time. Accordingly, the generated noise output quality and volume would also change. Thirdly, the high amplification requirements for amplifying the thermal noise make any system utilizing this technique subject to stray pickup of radiation fields and hum. In addition, the noise generation characteristics of a solid state device are normally uncontrollable. Therefore, high volume production is restricted since each noise generating semiconductor must be selected by preliminary tests. The uncontrollable parameters result in a low yield of usable units and consequently raise the costs of the system. Furthermore, after even the preliminary selection of the semiconductors, each unit must be individually adjusted for output level and sound quality. Accordingly, special variable resistance networks must be built into each system to permit such adjustment after assembly. Testingand adjustment time with trained personnel furthermore increases the cost of the system. Finally, using noise generating solid state devices restricts interchangeability without subsequent adjustment by trained and highly skilled personnel.